Traditionally, the whereabouts of animals have been controlled by erecting physical barriers such as walls, fences or gates at the site of a boundary the animal is to be prevented from crossing. Such barriers must not only be high enough to prevent the animal from jumping over and dense enough to prevent the animal from passing through any gaps but also substantial enough to withstand attempts by the animal to breach the barrier by physical force. The latter requirement is a serious limitation in that in addition to requiring considerable time and labor to erect, substantial physical barriers are often impracticable due to shortage of materials suitable for their construction.
Those limitations have been overcome to some extent by the development of avoidance-inducing physical barriers, of which, barbed wire and high voltage charged fences are well-known examples. Rather than relying solely on physical strength to defeat attempted breaches, animals quickly learn that contact with such barriers is associated with an aversive stimulus such as being shocked or pricked by sharp barbs. They therefore avoid repeated or sustained attempts to breach them. Physical barriers of the avoidance-inducing type have permitted the fencing of large areas with the expenditure of only a fraction of the time, effort and materials which had previously been necessary. However, avoidance-inducing physical barriers also suffer from some important limitations.
First, like all physical barriers, posts and wires or other above-ground structures are required to erect an avoidance-inducing physical barrier. In some applications, such as the confinement of household pets or guard dogs within a property line, these structures can be unsightly and are sometimes forbidden by deed restrictions or local regulations. Like all physical barriers, avoidance-inducing physical barriers are non-selective. A physical barrier sufficient to control the whereabouts of a particular animal also tends to impede the ingress and egress of persons or other animals except at locations where a gate may be provided. Moreover, the animal can traverse the barrier if the gate is inadvertently left open. Electrified or barbed wire fences intended for controlling the whereabouts of animals can also shock or injure persons, especially young children.
Various electronic systems which do not require the erection of above-ground barrier structures and which are at least somewhat selective in their operation are also known. In these systems, selectivity is achieved by equipping only the animal (or animals) whose whereabouts are to be controlled with an electronic unit capable of sensing when the animal moves into predetermined proximity of a defined boundary and then delivering one or more aversive stimuli to deter the animal from traversing the boundary. Such stimuli commonly comprise an electric shock either alone or in combination with an advance audible tone. A number of types of such electronic systems are known in the prior art. In most widespread use today are those of the "wire loop" type.
Various wire loop systems for controlling the whereabouts of animals are exemplified by U.S. Pat. No. 3,753,421 to Peck; U.S. Pat. No. 4,136,338 to Antenore; U.S. Pat. No. 4,733,633 to Yarnall, Sr. et al.; U.S. Pat. No. 4,745,882 to Yarnall, Sr. et al.; U.S. Pat. No. 4,766,847 to Venczel et al. and U.S. Pat. No. 4,937,695 to Giunta. In such systems, one or more continuous wire loops are routed along an arbitrary path to define a boundary. In some cases the wires are run above ground, in others they are buried. A controller connected directly to the loop generates an amplitude modulated (AM) signal which flows through the loop and causes an AM radio signal to radiate from the loop at a predetermined carrier frequency which is typically in the range of about 8 KHz to about 20 KHz. These systems operate by simple on/off keying. When a battery-powered receiver unit affixed to the animal receives an AM signal at a magnitude indicating close physical proximity of the animal to the boundary loop, the receiver initiates a generation of a tone and/or shock to deter the animal from crossing the loop. When an AM signal of such magnitude is not present at the receiver, no stimuli are applied to the animal.
Many AM wire loop systems which operate in this manner have been installed and are in operation at the present time. In certain of these systems, the animal's approach to the wire, as indicated by a received signal strength above a predetermined threshold, initiates application of a first and relatively mild aversive stimulus (such as generation of a tone) which terminates if the animal retreats from the boundary. If on the other hand, the animal moves closer yet toward the loop, a higher threshold of signal strength is exceeded and a stronger aversive stimulus such as an electric shock is administered in order to repel the animal from the boundary as defined by the location of the wire.
The low RF frequency AM modulation used in prior art wire loop systems provides a number of advantages. Their low frequency AM radio signals propagate through soil well enough to provide acceptable signal range aboveground. Low frequencies are also less likely to reflect from non-metallic obstructions which might otherwise create "shadows" or gaps in the boundary field. Another advantage of low frequency AM modulation is that if the wire loop is inadvertently c cut or breaks, the localized electrical field emitted across the break enables one to easily locate the break using an ordinary AM radio receiver. However, the low frequency AM wire loop boundary systems of the prior art also suffer from a number of disadvantages and limitations.
For example, AM systems are highly susceptible to electrical interference from a variety of common sources including motor vehicles, motor operated appliances, light dimmers, and television sets. Such interference can cause an animal to be shocked even when it is not near the boundary. This undesired shocking is not only unnecessary and inhumane if persistent, but can also confuse the animal and interfere with its being trained to associated the aversive stimulus with the intended boundary. In order to provide an acceptable margin against reception of false signals due to interference, AM systems of the prior art require a large received field intensity. Hence, the loop must be made to radiate a commensurately intense field.
Because a wire loop does not act as an efficient AM antenna at the low frequencies at which AM wire loop systems operate, a relatively large current, typically about two hundred (200) to about eight hundred (800) milliamps, must flow through the loop to generate a suitable boundary field. This not only consumes excessive power, making the system more expensive to operate but also precludes the possibility of powering the controller driving the loop using primary or backup batteries of reasonable size and cost. As a result, the controller must be mounted at a location, usually indoors, near an A.C. power outlet and an A.C. power outage renders the entire system inoperable. When this occurs, the entire boundary is breached not merely a localized portion of it making it much more likely that the animal will locate and exploit the breach. Also, since the closest point on the desired boundary may be some distance away from an available A.C. outlet, installation of wire loop systems is made more difficult, time consuming and expensive. The relatively large current flowing through the loop also creates a commensurately large low frequency electrical field. Some have suggested that such fields may pose health risks to humans or animals.
Installing the wire loop represents a substantial portion of the cost of a wire loop system. As noted above, a continuous length of wire must be run from the controller, around the desired boundary and back to the controller which is usually located at an indoor location remote from any point on the desired boundary. The wire must be installed around or through any intervening walls or other obstacles. Even with special equipment built for the purpose, it is not a trivial task to bury a wire loop encompassing the perimeter of a large property. Installation is further complicated by the necessity of twisting the loop wires together wherever they must pass through locations where no boundary field is desired. Since the currents in any twisted portions of the wires flow in opposing directions, their fields cancel sufficiently that the unit affixed to the animal does not initiate application of an aversive stimulus even when the animal is nearby. Thus, by twisting portions of the loop together, a boundary located remotely from the controller and/or one having two or more distinct portions lying physically separated from one another can be formed using a single loop of wire connected to a single controller.
Remote broadcast systems are another class of electronic system known in the prior art for controlling the whereabouts of animals. These do not require installing a loop of wire to define a boundary. Instead, a boundary is established by broadcasting an RF signal from a central location toward an intended outer perimeter boundary. The location of the boundary is defined based on the strength of that broadcast signal as sensed by a unit affixed to the animal. For example, U.S. Pat. No. 5,067,441 to Weinstein describes an animal restraining system including a radio frequency transmitter, a transmitting antenna located inside an area in which the animal is to be restrained and a collar unit worn by the animal. A coaxial cable is run between the transmitter unit and the transmitting antenna. When the animal strays from the transmitting antenna a distance sufficient to permit the signal strength received by the collar unit to fall below a predetermined level, a first aversive of stimulus, such as a beeping tone, is generated. If the animal strays further from the antenna by a distance sufficient to cause the signal strength to fall below a second predetermined threshold, a stronger stimulus such as a shock is administered to the animal to deter its departure from the area. A similar system is described in U.S. Pat. No. 4,898,120 to Brose.
A fundamental shortcoming of remote broadcast type systems for controlling animal whereabouts is that the collar unit worn by the animal does not detect proximity of the animal to a structure or object whose physical location reliably indicates the locator of the intended boundary. Instead, such systems rely on measuring signal strength as an indicator of the distance the animal from a transmitting antenna which may be located a considerable distance from the boundary. Consequently, that indication is not always reliable. One reason is that because such systems operate in a far-field regime, the magnitude of the field decreases only in proportion to the square of distance from the transmitting antenna and does not vary significantly over distances on the order of several feet. Another reason is that the strength of the received signal can change due to constructive and destructive interference generated by signal reflections, shadowing by metallic objects and other uncontrollable variations in local reception conditions. Since local reception conditions can fluctuate, the size, shape and location of the boundary loci at which stimuli will be administered can also fluctuate. For example, if the signal path between the transmitting antenna is temporarily altered by an automobile which pulls into one's driveway, the animal may receive a shock even if the animal remains within an intended perimeter boundary. Remote broadcast systems also tend to require significant amounts of electrical power and thus, like wire loop systems, do not lend themselves to battery operation. This disadvantage stems in part from the need to broadcast a sufficiently strong signal to the most remote portion of the boundary and renders these systems, like AM loop systems, vulnerable to A.C. power failures.
Remote broadcast systems are also limited with respect to the sizes and shapes of perimeter boundaries they can define. While generally circular boundaries or ones conforming to the radiation pattern of a particular antenna can be implemented, continuous perimeter boundaries encompassing jutting regions or other well defined irregularities would be extremely difficult, if not impossible to establish using a remote broadcast type system. Another limitation of such systems is that because signal strength values are not unique to individual locations within the field of the transmitter, they are not well suited for excluding an animal only from arbitrarily located discrete positions, such as the site of one's prized rose bush for example. While wire loop systems offer greater flexibility and predictability of boundary shape, they are subject to the problems and limitations described above.
Another significant limitation of both the wire loop systems and the remote broadcast systems described above is that they are only capable of defining boundaries whose positions remained essentially fixed. Prior art U.S. Pat. No. 5,241,923 to Janning described for the first time an animal whereabouts control system which, while suitable for defining discrete and/or continuous fixed boundaries, was also capable of defining boundaries which moved with a mobile agent such as a child or another animal so that a particular animal such as a dog could be kept separated from child or other animal while otherwise allowing both dog and child complete freedom of movement. The systems described in Janning '923 operated at relatively high frequencies such as 915 Mhz where high antenna efficiencies could be obtained so as to reduce power requirements sufficiently to permit the system to operate on battery power. In such a system, individual active or passive high frequency transponders could be placed singly to exclude the animal from a specific location or arranged in mutually spaced arrays to form closed or partially closed continuous perimeter boundaries of virtually any desired size and shape. The transponders could be encapsulated for burial in the earth outdoors or packaged for placement beneath carpets, furniture cushions or area rugs or near entrances to rooms from which an animal was to be excluded.
Being of small size, light weight, and completely self-contained, the transponders disclosed in Janning '923 required no external wiring. Those transponders could also be provided with adhesive backing or with clips, pins or other attachment devices for securing them at a desired fixed location or to a mobile agent such as a child, an automobile or another animal which one might desire keeping separated from a particular animal. Using a collar or other suitable means of attachment, there could be affixed to the latter-mentioned animal a battery powered unit incorporating a receiver coupled to a stimulator for delivering a tone and/or a shock to the animal being controlled. Mounted either remotely or within the battery powered unit affixed to the controlled animal was a transmitter which generated an incident signal in the form of intermittent bursts of continuous wave (CW) energy at 915 MHz. Upon receiving this incident signal, the transponders would generate a return signal. When the distance separating the animal from one of the transponders became sufficiently close, the return signal would be picked up by the receiver affixed to the animal and cause the stimulator to deliver an appropriate aversive stimulus. Optionally, one or more of the transponders could be provided with a delay line or surface acoustical wave device which could be used to encode the return signal so as to identify to the receiver the particular transponder from which the return signal is emanated so as to enable different animals to be subjected to different boundary constraints by a single system.
While representing a major breakthrough in the art due to their unprecedented flexibility, effective performance and simple, low-cost installation, systems for controlling animal whereabouts using transponders as disclosed in Janning '923 were subject to limitations. In particular, placement of transponders at certain locations, such as those adjacent or wholly or partially surrounded by metallic surfaces, sometimes resulted in blocked reflected or otherwise anomalous signal propagation manifested in undesired variations in distance sensitivity (i.e., boundary range) or the complete inability to establish an effective boundary at such locations.
Like all other known prior art systems (except for wire loop systems themselves), the systems described in Janning '923 also lacked the ability to function with a wire loop boundary. Accordingly, they could not be used to replace or upgrade existing AM wire loop systems of which, despite the disadvantages noted above, large numbers have been and continue to be installed in the field.